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NBS SPECIAL PUBLICATION 671

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

100

.U57

No ,671

1 1984

C.2
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities 2 — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science

THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Manufacturing

Engineering — Building Technology — Fire Research — Chemical Engineering2

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
Retroreflectance MAP Service for

Coefficient of Luminous Intensity

Kenneth L. Eckerle and Jack J. Hsia

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued February 1984
Library of Congress Catalog Card Number: 84-601001

National Bureau of Standards Special Publication 671

Natl. Bur. Stand. (U.S.), Spec. Publ. 671, 56 pages (Feb. 1984)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1984

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
TABLE OF CONTENTS

PAGE

I. Purpose of Program 1

II. Who Should Participate 4

III. Description of MAP Package

.

4

IV. Research Performed and Properties of Materials 4

A. Filters for Spectral Diagnostics 4

B. Retroreflectors 7

V. Instructions and Procedures 7

A. Measurements of Luminous Transmittance of the Seven Colored Glass Filters . . 7

B. Coefficient of Luminous Intensity of Prismatic Retroreflector 9

C. Coefficient of Luminous Intensity of Bead Sheeting Specimens 13

D. Results 13

VI. Estimation of NBS Uncertainty Bounds for Retroreflectors 13

A. Types of NBS Uncertainties for Retroreflectors 13

(1) Geometrical 13

(2) Source and Receiver Apertures 14

(3) Linearity and Noise for Coefficient of Luminous

Intensity Measurements 14

(4) Realization of Illuminant A and CIE Photopic V(A) for Source

and Receiver 14

(5) Transfer Sample 14

(6) Material 14

B. Summary 14

VII. Pilot MAP Studies for Retroreflectors 15

A. Participating Laboratories 15

B. Results of the MAP Procedure 15

C. Summary 17

VIII. Colored Filters for Checking the Spectral Quality of Retroref lectometers 17

A. NBS Associated Uncertainties for the Colored Filters 17

B. Pilot Run for MAP Colored Filters 19

IX. Conclusions 19

Acknowledgments 19

Illustration A 32

iii
Table of Contents (continued)

PAGE

Illustration B 33

II 1 ustration C 35

References 36

Appendix A 37

Appendix B 40

Appendix C 41

iv
LIST OF TABLES

TABLE NO. PAGE

1. NBS uncertainties in angular parameter values 20

2. Total NBS Uncertainty P NB$ for Master Bead Sheeting Retroreflectors 21

3. Total NBS Uncertainty P NBS

for Master Prismatic Retroreflector 22

4. Other uncertainties associated with measurements of R at NBS 23

5. Uncertainties in the parameters a, Bi , £2, and e for the

measurements performed by laboratories 1 and 2 24

6. Other uncertainties associated with measurements of R for

the participating laboratories 25

7. Laboratory uncertainties P^g* total uncertainty of the intercomparison U, and differences from NBS measurements D 26

8. Laboratory uncertainties, P^g' total uncertainty of the

intercomparison U, and differences from NBS measurements D 27

9. Laboratory uncertainties P^g* total uncertainty of the intercomparison U, and differences from NBS measurements D 28

10. Laboratory uncertainties P|_^g> total uncertainty of the intercomparison U, and differences from NBS measurements D 29

11. Values for Master Set of Filters Associated with NBS 30

12. Uncertainties of the participating laboratories' measurements 31

Appendix C

1. Results of pilot intercomparison with Laboratory 1 43

2. R

L ^B are the results of the participating laboratory, with ^[35^

AFTER

and R^bs the NBS measurements made before and after the

participating laboratory 44

3. Results of pilot intercomparison with Laboratory 1 45

4. Laboratory uncertainties P|_^B

> total uncertainty of the intercomparison U, and differences from NBS measurements D 45

5. Laboratory uncertainties, P|_ AB

> total uncertainty of the intercomparison U, and differences from NBS measurements D 47

6. Other uncertainties associated with measurements of R for the

participating laboratories . . 48

7. Uncertainties of the participating laboratories' measurements 48

v
LIST OF FIGURES

FIGURE NO. Page

1. Schematic of Geometry 3

2. Mechanical drawings of retroref lectors and mounts 5

3. Mechanical drawing of filters and mounts 6

4. (a) Sketch showing removal of the top of MAP package container 8

(b) Attachment of retroreflectors to their shipping plates

and showing colored filters 8

5. Two methods for measuring the luminous transmittance of the seven colored glass filters 10

6. Orientation of lettering on the prismatic retroreflector (RR)

with respect to source (P) and receiver (R) 11

7. Sketch showing engravings on retroref1 ector holder every 90° for

the purpose of setting the rotation angle, e, equal to zero 12

8. Sample control chart obtained from data collected using the

master prismatic retroref! ector 16

9. Graph of data shown in Table 8 illustrating D versus various geometries .... 18

Al . Apparatus for measuring detector linearity 38

vi
RETROREFLECTANCE MAP SERVICE FOR COEFFICIENT OF LUMINOUS INTENSITY

Kenneth L. Eckerle and Jack J. Hsia

This publication is written for those participating

in the retroreflectance measurement assurance program

(MAP) service provided by the National Bureau of Standards.

This service is to verify the accuracy of measurement of coefficient of luminous intensity (R). This paper presents

the techniques and procedures that are pertinent to participating in the MAP service, as well as a detailed explanation

of the error analyses. Uncertainties for both retroreflectance and luminous transmittance were determined from two pilot

studies carried out with the assistance of two industrial

laboratories and by research performed on the elements of the

MAP package.

Key words: bead sheeting retroref 1 ector; coefficient of

luminous intensity; diagnostic filters; error estimation;

experimental design; luminous transmittance; Measurement

Assurance Program; prismatic retroref 1 ector; uncertainty

estimation.

I. Purpose of Program

The coefficient of luminous intensity, R, for a retroref lector is defined as [1]

R = I/E

The quantity R is obtained by dividing the luminous intensity I of the retroref lector in the

direction of observation by the illuminance E^ at the retroref 1 ector on a plane perpendicular to the direction of the light.

If the same receiver is used to measure both I and E , this equation becomes

R = (m 1 /m 2 )d 2

where mi is the signal with the receiver at the observation position, m2 is the signal with the

receiver in the sample position, and d is the distance between the receiver aperture and the

sample. Ideally, coefficient of luminous intensity is defined for a retroref 1 ector which is a point source. In our case, the retrorefl ector sizes are very small compared to the observation

distance, d; and they provide a good approximation to a point source. Errors caused by finite

size of the retroref 1 ector are discussed in reference [1].*

R is defined for both colored and colorless retroref lectors. The light source is chosen

to correspond to CIE Illuminant A [2] and the detector is chosen to correspond to the CIE defini

tion of V(X) [2].

The new geometrical parameters used to define directions were published recently [1,3].

These include the observation angle, a, the entrance angles, 3j and 3 2 > anc' tne rotation angle,

*Figures in brackets indicate the literature references at the end of this paper.

1
In figure 1, a and 3 are shown with &i=0 (the component of 3 in the observation plane) and 32=3

(the component of 3 in the entrance plane). The parameter e is the angle of rotation of the

reflector about its reference axis.

The purpose of this Measurement Assurance Program (MAP) is to check how well a given laboratory can measure R and to tie that laboratory to the National Standardization Program. This is achieved in a simple and direct method which will simultaneously check measurements and procedures

by means of a package of ret roref 1 ectors and filters to be measured by the participating laboratory.

The MAP package contains two types of colorless retroref 1 ectors--bead sheeting and prismatic

or cube corner. There is a high intensity specimen (encapsulated lens) and an engineering grade

specimen (enclosed lens) of bead sheeting [4]. The prismatic or cube corner reflector is of the

two-orientation hexagon cube type of plastic retroref lector [4]. The primary purpose of using

these colorless retroreflectors is to check how well the geometrical parameters of the system can

be set.

Also included in the MAP package are seven filters which are to be used for diagnostic tests

of the spectral quality of the apparatus. These tests are accomplished by insertion of each

filter between source and receiver to measure the luminous transmittance, Y, of the filters.

/

S A (A)x(X)V(X)dX

Y =

/ S

ft (X)V(X)dX

where S^(X) is the spectral power distribution of CIE Illuminant A, V(X) is the CIE photopic

response curve, and x(X) is the spectral transmittance of the filter. The integration normally

includes the visible spectrum from 380 to 770 nm [2]. These filters are usually not used to obtain color correction factors but to show conformance of the source to CIE Illuminant A and

conformance of the receiver to V(X).

The success of these tests depends on linearity of the measurement system which can be checked independently by a method described in Appendix A.

Obviously, all values of the various geometrical and spectral parameters cannot be checked in

a cost effective way. Consequently, it is up to the participant to correlate the results of this

program with results for other parameter values such as the measurement of R for an observation

angle of 0.33°--a common value in Europe. For a reasonably thorough check of the test system it

is recommended that the participant use the complete MAP package. However, for those whose

facilities can accommodate only one type of retroref lector, bead sheeting or prismatic, we will

make available that one type of retroref lector and/or the filters as a limited service.

Measurement of even the complete MAP package achieves only part of the goal of a MAP service.

To fully benefit from the MAP procedure, we suggest that the participant have on hand several

check standards to be measured while he is measuring the MAP package. These check standards can then be measured periodically to determine any gross error in measurement procedure, and a control

chart can be constructed. A control chart is a plot of measurement result versus time, and

normally the measurment process is considered to be under control if measurements fall within

2
Figure 1. Schematic of Geometry. P - Source. R - Receiver. A - Observation Plane.

B - Entrance Plane. RR - Retroreflector. C - Reference Axis.

D - Illumination Axis. E - Observation Axis. 6 - Entrance Angle,

a - Observation Angle. T - Source to Receiver Distance.

3
±three standard deviations from the mean. For retroref 1 ectance measurements where geometric errors

are large, the standard deviation obtained after changing geometrical parameters may be large

compared with that obtained from repeated measurements without changing the apparatus. Thus, the

total variation for a given instrument can be obtained only by repetition over a period of time

and re-alignment of the experimental apparatus.

II. Who Should Participate

Secondary and test laboratories, corporate laboratories, manufacturing laboratories, state

laboratories, or others may benefit. The laboratory must, of course, be willing to perform the

required measurements with the MAP package. Of equal importance is the capability to perform

independent tests such as the linearity measurement and the ability to precisely align the

equipment for control of the geometrical parameters. The importance of using check standards and

control charts to maintain control of the measurement process after the MAP package has been

returned to NBS cannot be overemphasized.

III. Description of MAP Package

The contents of the MAP package were itemized in Section I above. The filters and

retroreflectors have been mounted in such a fashion that the participant should not have to modify

them in order to perform the measurements. The retroreflectors are in black holders which have

holes conveniently placed in case an adapter plate is necessary. (See figure 2 for dimensions.)

The colored filters are mounted in holders with a convenient mounting rod. (See figure 3 for

dimensions.

)

Measurements made on a set of these reflectors and filters and the error analyses are described in references [5] and [6].

IV. Research Performed and Properties of Materials

Details of research and material properties will be found in references [5] and [6]. They

will be summarized briefly.

We have been aware of the problem that the measurements of many laboratories differ by a factor of 2 or more from each other [4]. Although we have not had the opportunity to work with

these laboratories to try to determine the exact cause of these discrepancies, we have tested our instrument and evaluated bounds on the uncertainties due to spectral and geometrical errors. (See

section VI below.) We have evaluated the bounds on the uncertainties due to various other causes

and have found that some of these can be significant.

A. Filters for Spectral Diagnostics

A set of seven filters plus a plastic ultraviolet -absorbing filter is included to check

how well a test instrument can measure luminous transmi ttance and thus the spectral effect on measurements of coefficient of luminous intensity of colored retroreflectors. The seven filters

include vivid blue, deep yellowish green, vivid orange yellow, vivid yellow, vivid reddish orange,

vivid red, and black (infrared transmitting). The purpose of the black filter is to check for the

presence of any radiation in the near infrared. The spectral transmittance of a master set of

filters was measured using a high accuracy reference spectrophotometer [7,8,9]. These spectral

transmittance data are converted to luminous transmittance by calculations using the defined

values of CIE Illuminant A and CIE V(A) [2], The systematic errors in the values obtained with a

4
Figure 2. Mechanical drawings of retroref lectors and mounts.

Top - prismatic and bottom - sheeting. Certain material is identified

only to define the procedure. It implies in no way endorsement by the

National Bureau of Standards or that it is best for the procedure.

5
Figure 3. Mechanical drawing of filters and mounts.

6
commercial spectrophotometer are determined by comparison with values obtained with the reference

spectrophotometer using a master set of filters. The commercial instrument is then used to calibrate duplicate sets of filters. The chromaticity of these filters is similar to those of commonly used retroref lectors. The filters are to be measured in one of two methods described in

section V.A.

B. Retroreflectors

We evaluated the effect on R due to uncertainties in the geometrical parameters a and Bi,

as well as the angular apertures, 6 and y> of the source and receivers for the bead sheeting

retroreflectors at a test distance of approximately 15 m. We also measured the effects on R of uncertainty in a, B l5 B 2 , ancl e f° r the prismatic retroreflectors at a distance of approximately

30 m. Finally, the pressure and temperature effects on R for both the bead sheeting and the

prismatic retroreflectors were measured over the temperature range of 15°C to 32°C and the

pressure range 0.0085 kilopascal (0.85 bar) to 0.0125 kilopascal (1.25 bar). The results enable a prediction of bounds on R if the participant can supply his uncertainty for the geometrical parameters. The pressure-temperature results permit an estimate of the upper bound on the uncertainties. The results of these tests are summarized in references [5] and [6],

V. Instructions and Procedures

The Measurement Assurance Program (MAP) package should be handled carefully as the component

pieces are fragile. Care must also be taken not to touch the optical surfaces of the components

of the packages.

As we have described above, the MAP package consists of seven colored glass filters, a plastic

UV cutoff filter, one prismatic ret roref 1 ector , and two retroref 1 ecti ve sheeting samples (one high

intensity and one engineering grade). The retroreflectors with holders have been fastened to aluminum plates for shipping purposes. The box may be opened and the components may be detached

as shown in figure 4. An identifying letter (A, B, C, etc.) is engraved on the back of each

piece. None of the optical surfaces should be cleaned except for a light brushing with a clean

camel's hair brush if dust is present.

It is strongly suggested that a linearity check such as the light addition method using

retroreflectors (see Appendix A) or the two-source method be made before proceeding with the MAP measurements. Also geometrical calibration checks of angles and distances should be done, e.g.,

such as those described in reference [10].

The order in which the following measurements are performed is not important.

A. Measurements of Luminous Transmittance of the Seven Colored Glass Filters.

The measurements of luminous transmittance are to be made with the retroref lectometer

under test. The source is to be CIE Illuminant A and the receiver is to have the response of the

CIE V(A) function. The transmittance thus measured is called the luminous transmittance (Y). It

is defined as a ratio. The numerator is the signal with colored filter and plastic UV cutoff

filter in place (Sj) minus signal with source blocked (Z^. It is to be divided by the denominator which is the signal with only plastic UV cutoff filter in place (S2 ) minus signal with source blocked (Z 2 ). Thus, Y = {S x -l x )/ (S 2 -Z2 ) . The arrangement for the measurements is either

the "straight through" method shown in figure 5 or the "white diffuse reflector" method also shown

7
REMOVAL OF LID BY UNFASTENING EIGHT SCREWS

COLORED GLASS

FILTERS IN HOLDERS

BEAD SHEETING RETROREFLECTOR

FASTENED TO SHIPPING PLATE

PRISMATIC

RETROREFLECTOR

FASTENED TO

SHIPPING PLATE

BEAD SHEETING RETROREFLECTOR FASTENED

TO SHIPPING PLATE

TO DETACH RETROREFLECTORS FROM SHIPPING PLATES REMOVE TWO FASTENING SCREWS.

Figure 4. (a) Sketch showing removal of the top of MAP package container

and

(b) attachment of retroreflectors to their shipping plates and

showing colored filters.

3
in figure 5. BaSO^ or pressed polytetrafluorethylene powder [11] are recommended as diffusers if the latter method is used.

The colored filters are to be inserted with their surface perpendicular to the light beam.

The clear plastic is to be inserted in the beam at an angle to eliminate multiple reflections with

the colored filters and left there for all subsequent readings for luminous transmittance. The

distance between source and receiver is not critical. The luminous transmittance desired is that

of the colored filter, not the combination of colored filter and clear plastic filter, so only the

colored filters are to be removed for each measurement. The only purpose of the clear plastic

filter is to filter out ultraviolet light, (the plastic filter was determined not to fluoresce),

which could cause the colored glass filters to fluoresce. It is suggested that at least three

measurements be made on each colored glass filter and the standard deviation* be computed. A data

sheet is enclosed to report the average and the standard deviation (see illustration A). The

individual value of Y should be reported on separate sheets of paper.

B. Coefficient of Luminous Intensity of Prismatic Retroref 1 ector.

The observation angle is a, the two components of the entrance angle are 3i and 32, and

the rotation angle is e as defined above in Section 1. For a more detailed definition, see reference [3]. The relationship between the CIE system and other systems of coordinates is also

given in reference [3].

The clear prismatic (cube corner) reflector as shown in figure 1 is to be measured at an observation distance of approximately 30 m with source and receiver apertures of approximately

3 minutes of arc if possible. The orientation of the lettering with respect to source and

receiver is to be as shown in figure 6. For a fine adjustment engravings have been placed on the

holders every 90 degreees in groups of three as shown in figure 7. The central engravings are to

be used with a plumb bob to set the rotation angle e = 0. The two engravings on either side

denote an error of ±1 degree, e should be set with retroref 1 ector vertical so the plumb bob will

not be influenced by friction between the holder and the plumb bob string. It is suggested that

at least three measurements be made and the standard deviation computed. R is normally reported

in candela/lux (cd'lx -1

). These units may be reduced to (m 2 «sr

-1

)' If other units are used and a conversion is made, the conversion factor is to be reported. The measurements are to be reported

*The standard deviation (S.D.) may be computed from the formula,

S.D. =

where x. are the individual values, x is the arithmetic mean, and N is the number of independent

measurements. Another quantity which will be used is the standard deviation of the mean (standard

error) (S.E.), defined as

S.E.

/N

9
RECEIVER SOURCE

ho

COLORED FILTER PLASTIC FILTER

WHITE DIFFUSING

PLAQUE

i l

COLORED FILTER PLASTIC FILTER

SOURCE

RECEIVER

Figure 5. Two methods for measuring the luminous transmittance of the seven colored glass filters.

10
11
RECEIVER SOURCE

Figure 7. Sketch showing engravings on retroreflector holder every 90° for

the purpose of setting the rotation angle, e, equal to zero.

12
at the observation angles and the entrance angles on the report sheet (see illustration B.). The

individual values of R used to compute the mean are to be reported on separate sheets of paper.

C. Coefficient of Luminous Intensity of Bead Sheeting Specimens.

The geometrical angles are the same as in section B above. The two sheeting samples are to be measured at an observation distance of approximately 15 m with source and receiver apertures

of approximately 3 minutes of arc if possible. The identifying letter on the back of samples

should be at approximately 90 degrees with respect to the plane formed by the source, receiver,

and sample position. Its placement is not critical. Along with the identifying letter, the

letters HI and EW refer to high intensity and engineering white, respectively. Again, the results

for R are to be reported in candela/lux along with the conversion factor if a conversion is made.

The measurements are to be reported at the observation angles and the entrance angles on the

report sheet (see illustration C). The individual values of R used to compute the mean are to be reported on separate sheets of paper.

D. Results

After the measurements and the forms are completed, it is advisable to telephone the

authors to discuss your results.* A preliminary discussion may uncover a possible error and the

need for further measurements.

VI. Estimation of NBS Uncertainty Bounds for Retrorefl ectors

The uncertainties of retrorefl ectometer measurements come from three sources: NBS

uncertainties associated with values assigned to the MAP package, participant uncertainties, and

uncertainties due to environment and sample interaction. The latter two types are presented in Section VII below. Further, repeated measurements without changing the apparatus show that the

NBS random error is small relative to the systematic errors. A large fraction of the latter arise

when the retrorefl ector is rearranged and realigned for making measurements with different

measurement parameters. Consequently, the random and systematic errors are discussed together.

Also, the geometrical errors are presented for each angular setting, and the other errors are treated as independent of angular setting.

A. Types of NBS Uncertainties for Retrorefl ectors

The sources of error are classified in six categories, as follows:

(1) Geometrical

The relevant angular parameters for each of the th ree types of retrorefl ectors have

been varied according to a statistical design [5,6], Polynomial approximations obtained from the

resulting measurements yield coefficients which may be used to estimate measurement uncertainty

bounds if the uncertainties of the angular parameters can be estimated [5,6]. The uncertainties

in the angular parameters depend on instrumental qualities such as vernier scale resolution,

distance measurements and stepping motor resolution. Also, accuracy in initialization of these

*Radiometric Physics Division

National Bureau of Standards

Washington, DC 20234

(301) 921-2791

13
parameters must be taken into account. We estimated upper bounds for the uncertainties (U^, ,

Ug^, and U

£

) in our angular parameters which are given in Table 1. The geometrical uncertainties

are obtained from eq. (5) in reference [5] and eq . (4) in reference [6], These estimations are listed in Table 2 for the bead sheet retroreflectors and in Table 3 for the prismatic retroref 1 ectors

.

(2) Source and Receiver Apertures

The source and receiver apertures must be specified. Normally, we calibrate with

source and receiver apertures of 3 minutes of arc. The centroid of the source and receiver

apertures will be shifted if the source and/or the receiver are not uniform. For our case we

estimated an 8 percent uncertainty in aperture centroid location, which yields the approximate

uncertainties for the bead sheeting listed in Table 4. These estimates are based on the data in

reference [5]. Corresponding uncertainties for the prismatic retroreflectors are also listed in Table 4 and are based on the data in reference [12].

(3) Linearity and Noise for Coefficient of Luminous Intensity Measurements

The uncertainty in the linearity correction and the random noise in the measurement

are based on repeated measurements. We estimate the combination to contribute 1 percent as listed

in Table 4.

(4) Realization of Illuminant A and CIE Photopic V(X) for Source and Receiver.

The color temperature of the source and the receiver response were calibrated as described in reference [10]. Using the spectral reflectance curve for a commonly used white or nearly colorless retroref 1 ector, we estimated the uncertainty bounds in R for our source and V(A)

receiver to be less than 0.1 percent as listed in Table 4.

(5) Transfer Sample

A transfer sample or working standard was used. This procedure is described in

reference [10]. Due to the large dynamic range required for this calibration procedure and the

small signal in the usual receiver position, we bound this uncertainty as ±2 percent based on the

range of repeated measurements and list this uncertainty in Table 4.

(6) Material

We cannot say yet with certainty whether or not the samples in the MAP package have

changed during the period of time which has elapsed since we began observing them. However, our results indicate that the samples do not change measurably during the time required for a MAP service to be completed. The samples have been stored under laboratory conditions, and we will

continue to monitor their R values.

B. Summary

We choose to estimate the total uncertainty P^q$ as the arithmetic sum of the geometrical

uncertainty plus the other uncertainties in Table 4. This estimation yields a higher uncertainty

estimate than adding the errors as the square root of sum of the squares of individual errors (quadrature). Since the method chosen is more conservative, we can identify a bias in the results

of a participating laboratory with more certainty. In many cases, this final sum is an uncertainty much larger than we had anticipated. The total estimated upper bound Pm

R

<- for the

14
uncertainties for the bead sheeting are given in Table 2 and for the prismatic retroref 1 ector in

Table 3.

The control chart is basic to the concept of a MAP procedure. Consequently, NBS is accumu- lating data to establish a history of the NBS measurement process. A sample control chart is shown in figure 8. It shows measurement events in chronological order with delimiters to show

control of the measurement process. The delimiters are ±3 standard deviations. Since the data

are obtained from measurements which result when the apparatus is realigned, they contain

geometrical uncertainties. The data are obtained from measurements on the Master Prismatic

Retroreflector with a = 0.5°, g x = 10°, e 2 = 20°, and e = 0°. Each point represents a single

measurement. Comparison of the delimiters shows that they are consistent with the values of P^g^

in Tabl e 3.

VII. Pilot MAP Studies for Retroref 1 ectors

Two pilot runs using different MAP packages were undertaken by NBS in cooperation with two commercial laboratories. Each laboratory submitted estimates of its uncertainties in angular

parameters (Table 5). These permit estimation of geometrical uncertainty bounds due to variation

of a, f$i, 32» and e in the participating laboratory. The uncertainties in the parameters must take into account such things mentioned in Section VI. A. 1 above. The linearity was checked by the

participating laboratory and the random error was established by repeated measurements. The color

temperature of the source and the response combination of the detector were checked using the

colored filters in the MAP package. As will be seen below, the uncertainty due to the source and

detector on the colorless retroreflector will be small. The laboratories did not state any additional error due to the use of a transfer standard.

A. Participating Laboratories

The two laboratories participating will be designated "Laboratory 1" and "Laboratory 2."

Their estimates of uncertainties for a, e ls B 2 > and e are given in Table 5. Other parameters for

these measurements are given in Table 6.

In addition to the uncertainties of NBS and the participating laboratories, we estimate

possible differences due to temperature and pressure on bead sheeting and prismatic retroreflectors. These estimations indicate that for the normally encountered temperatures and

pressures one can expect only small changes in R [5]. For temperatures of 25 ±4°C and pressures

of 0.01 ±0.001 kilopascal (1 ±0.1 bar), this uncertainty (S) for the bead sheeting is ±0.5 percent

and for the prismatic retroreflector is ±1.5 percent.

B. Results of the MAP Procedure

Results are listed in Tables 7 through 10. The quantity, AR/R, is the predicted relative

geometrical uncertainty obtained from the individual assessments of U a , 1)^ , U^, and U

£ (the

uncertainties of the angular parameters) where appropriate. The columns headed by 3 x 5R p/R

p are three times the standard error divided by the mean. The other columns are headed by letters which

define the following:

15
0.40

0.38 Mean + 3X Standard Oeviation

o

0.36

0.34

0.32

O

O

- O

O

O

Mean

o

o

0.30 Mean — 3X Standard Oeviation

0.28 1 1 1 Ji

Figure 8.

2 3 4 5 6

MEASUREMENT NUMBER

Sample control chart obtained from data collected using

the master prismatic retroreflector.

16
L = Sum of the uncertainties for aperture centroid, linearity, and

spectral quality listed in Table 6 in percent.

PLAB

= Total uncertainty in percent attributed to the participating

laboratory

= AR/R + 3 6R

p /Rp + L

U = total uncertainty in percent for the comparison between the

individual laboratories and NBS including uncertainties S due to temperature and pressure changes.

= PNBS

+ PLAB+ S

B = Half of the difference in percent between the two measurements made by NBS. The first measurement was made before

that of the participating laboratory and the second measurement was made after the participating laboratory. Changes

in the material are indicated only if B is in excess of P^sD = Difference in percent between the value of R obtained by the

participating laboratory and the average of the "before" and

"after" measurements made by NBS.

C. Summary

Column B indicates that there were no gross changes in the MAP materials. Comparison of

U and D indicate that within the estimated uncertainty of the MAP measurements, the laboratories

can measure as well as their stated uncertainties permit. This is easily seen, for example, from

figure 9 where the data from Table 8 are presented graphically. D is the ordinate with brackets

denoting ±U. The abscissa represent the various geometries, which are labeled. In all cases, the

uncertainty brackets contain the value D = 0.

VIII. Colored Filters for Checking the Spectral Quality of Retroreflectometers

The reference values of Y for the master set I are given in Table 11 along with their ISCCNBS color names. The values are calculated from the measured transmittance, CIE Illuminant A source and CIE V(X) receiver.

A. NBS Associated Uncertainties for the Colored Filters

These uncertainty bounds are presented in tabular form in Table 11, along with the

filter identification and standard values for the master set. The component uncertainties are designated A, E, Caster and

^MASTER* ^ he Q uantity , A » is a conservative estimate of the

uncertainty in the value of Y obtained with the reference spectrophotometer. The values listed

for E are the magnitudes of the difference between the Y values obtained with the reference

spectrophotometer and those obtained using a commercial spectrophotometer. The values for CASTER

17
+30

+20

4r I] 0

^ 0

-10

-20

-30

-40

-50

-nO^So°cJwowtVJOCV|WoW(M

o 2 Ho o L o o cf 2 - + o o +. o o

I + 9. +. o. t + om t + q. T +, <d +

m iqiqiocvjcsicsiinioiocsicvicsiioioiocvjcsjcsi 2222222222^2222^22

+ +

Q

(<X,p,,p

2

) As Identified

Figure 9. Graph of data shown in Table 8 illustrating D versus various geometries.

13
are the magnitudes of half of the differences between NBS measurements made before and after the

participating laboratories made their measurements. Finally,

U MASTER

= A + E + ^MASTER

These uncertainties are derived from measurements on the master filters which remain at NBS at all times

.

B, Pilot Run for MAP Colored Filters

We assume that the same errors A + E apply to the commercial spectrophotometer measurements of both the master filters and the MAP filters. The values of 3 6Y

p in Table 12 are three

times the standard error derived from values reported by the laboratories. Again, C designates

the magnitudes of half of the differences between the NBS measurements made before and after the

participating laboratories. The bound on total uncertainty, U, is defined as

U=3x6Y

p +C+A+E .

The difference between the average of the measurements made by NBS before and after the

participating laboratory and the values reported by the participating laboratories is designated

by D. Finally D is compared to U to see if a significant difference exists. In some cases, D is greater than U, indicating a real difference between NBS and the participating laboratories.

However, these differences are not large and indicate that the spectral quality of the instruments

is satisfactory for most specifications.

IX. Conclusions.

The results obtained with the MAP packages indicate that they perform as they were designed.

Using this MAP package, a participating laboratory may assess the accuracy of its measurements.

Characterization of the MAP package has led to more accurately known NBS values for the various

parameters. Since there now exists a large disagreement between some laboratories [4], use of this MAP package can assist in the achievement of more accurate retroref 1 ectance measurements and

better agreement among laboratories.

Acknowl edgments . This project is supported by the Office of Physical Measurement Services

(B. C. Belanger, Chief) and was coordinated by L. J. Kieffer and R. K. Kirby. The authors

wish to thank V. R. Weidner for measurement of the colored filters using the commercial spectrophotometer. We greatly appreciate the previous help with the statistical analysis used for

the program, and for the consultation provided for error analysis in the present paper by

W. S. Liggett, Jr., of the Statistical Engineering Division. We also wish to thank the NBS Calibration Advisory Group subcommittee for their critical review of the manuscript. This

subcommittee was composed of T. Vorburger, M. C. Croarkin, and M. Young.

19
TABLE 1

NBS uncertainties in angular parameter values.

Bead Sheeting

U a

= ±0.008°

u 3l

= ±0.1°

Prismatic Retroref 1 ectors

U

e = ±1.0°

U

g

= ±0.10°

U 62

= ±0.3°

U a

= ±0.004°

20
TABLE 2

Total NBS Uncertainty P^QS ^ or Master Bead Sheeting Retroref lectors

Previously published values of R, predicted geometrical uncertainty (AR), and

relative geometrical uncertainty (AR/R). The uncertainty, P NBS» ^ s tne sum °f

the geometrical uncertainty and the values in Table 4.

High Intensity White Sheeting

a 01 R AR AR/R P NBS

grees

)

(degrees

)

(cd/lx) (cd/1 x) {to)

0.2 -4 3.8546 ±0.05 27 ±1.37 ±4.67

0.2 20 3.5722 ±0.0493 ±1.38 ±4.68

0.2 40 2.3333 ±0.0331 ±1.42 ±4.72

1.5 -4 0.1956 ±0.0030 ±1.52 ±4.82

1.5 20 0.1865 ±0.0028 ±1.52 ±4.82

1.5 40 0.1362 ±0.0018 ±1.34 ±4.64

Engi neeri ng White Standard

0.2 -4 1.4789 ±0.0166 ±1.12 ±4.32

0.2 20 1.1474 ±0.0140 ±1.22 ±4.42

0.2 40 0.3505 ±0.0031 ±0.90 ±4.10

1.5 -4 0.1295 ±0.0013 ±1.04 ±4.24

1.5 20 0.1199 ±0.0013 ±1.12 ±4.32

1.5 40 0.0771 ±0.0009 ±1.16 ±4.36

21
TABLE 3

Total NBS Uncertainty P NBS for Master Prismatic Retroref 1 ector

Values of R, predicted geometrical uncertainty (aR), and relative geometrical uncertainty (AR/R). The quantity, R, is equal to Y2 in reference [6], and represents an

average value of the measurements. The uncertainty, P^gs* 1S the sum °f tne tipper

bound for geometrical uncertainty and the values in Table 4 with e=0°.

a 01 32 R AR AR/R P NBS

iegree

)

(degree) (degree

)

(cd/1 x) (cd/1 x) (%) {%)

0.5 0 0 1.023 ±0.0233 ±2.28 ±5.88

0.5 -10 0 0.937 ±0.0218 ±2.33 ±5.93

0.5 +10 0 0.923 ±0.0264 ±2.86 ±6.46

0.2 0 0 5.600 ±0.142 ±2.53 ±6.13

0.2 -10 0 4.759 ±0.144 ±3.03 ±6.63

0.2 +10 0 4.785 ±0.154 ±3.22 ±6.82

0.5 0 -20 0.520 ±0.0559 ±10.8 ±14.4

0.5 -10 -20 0.388 ±0.0451 ±11.6 ±15.2

0.5 + 10 -20 0.392 ±0.0362 ±9.23 ±12.8

0.2 0 -20 2.456 ±0.217 ±8.84 ±12.4

0.2 -10 -20 2.201 ±0.247 ±11.2 ±14.8

0.2 +10 -20 2.186 ±0.214 ±9.78 ±13.4

0.5 0 +20 0.487 ±0.0309 ±6.34 ±9.94

0.5 -10 +20 0.375 ±0.0281 ±7.50 ±11.1

0.5 +10 +20 0.415 ±0.0280 ±6.74 ±10.3

0.2 0 +20 2.910 ±0.123 ±4.22 ±7.82

0.2 -10 +20 2.315 ±0.162 ±7.02 ±10.6

0.2 +10 +20 2.586 ±0.128 ±4.74 ±8.34

22
TABLE 4

Other uncertainties associated with measurements of R at NBS

Engineering Grade High Intensity Prismatic

White White Clear

Uncertainty in

Aperture Centroid ±0.1% ±0.2% ±0.5%

Linearity and

Noise ±1.0% ±1% ±1,

Col or Temperature of Source

and Photopic ±0.1% ±0.1% ±0.1%

Response of Recei ver

Transfer Sample ±2.0% ±2.0% ±2.0%

Sum ±3.2% ±3.3% ±3.6%

23
TABLE 5

Uncertainties in the parameters a, g ls 3 2 » and e for the measurements performed by laboratories 1 and 2.

Laboratory 1

Bead Sheeting Prismatic

U = ±0.005° U = ±0.005°

a a

U

p

= ±0.10° Up = ±0.1°

u

e

= ±2<

Laboratory 2

Bead Sheeting Prismatic

U a

= ±0.002° U a= ±0.002'

Up = ±0.25° Up = ±0.25°

Up

2

= ±0.25°

U

e = ±0.5'

24
TABLE 6

Other uncertainties associated with measurements of R for the

participating laboratories.

Laboratory 1

Engineering Grade High Intensity Prismatic

Aperture Centroid

(Source and Receiver)

Li nearity

Color Temperature of

Source and Photopic

Response of Receiver

White

±0.1%

±2.0%

±0.1%

2.2%

White

±0.2%

±2.0%

±0.1%

2.3%

CI ear

±0.5%

±2.0%

±0.1%

2.6%

Laboratory 2

Aperture Centroid

(Source and Receiver]

Li nearity

Color Temperature of

Source and Photopic

Response of Receiver

±0.7%

±1.5%

±0.1%

±1.5%

±1.5%

±0.1%

±0.5%

±1.5%

±0.1%

2.3% 3.1% 2.1%

25
TABLE 7

Laboratory uncertainties p

|_/\g» total uncertainty of the

i ntercomparison U, and differences from NBS measurements 0.

The other column headings are defined in section VII-B.

Laboratory 1

High Intensity White

31 AR

(%)

3x6R L P LAB U B D (degree) (degree) R Rp (%) (%) (%) (%) (%)

0.2 -4 ±0.86 ±0.421 ±2.3 ±3.581 ±8.751 0.997 -1.604

0.2 20 ±0.89 ±0.272 ±2.3 ±3.462 ±8.642 0.900 -1.159

0.2 40 ±1.052 ±0.432 ±2.3 ±3.784 ±9.004 0.505 0.484

1.5 -4 ±0.95 ±0.085 ±2.3 ±3.335 ±8.655 0.785 -2.759

1.5 20 ±0.98 ±0.530 ±2.3 ±3.810 ±9.130 0.262 -2.672

1.5 40 ±0.97 ±0.124 ±2.3 ±3.394 ±8.534 1.069 -2.470

a 3i AR

(yo

) (degree ) (degree) 5

0.2 .4 ±0.72

0.2 20 ±0.88

0.2 40 ±0.90

1.5 -4 ±0.68

1.5 20 ±0.74

1.5 40 ±0.89

Engineering White

3x6R n —/(%)

L

(%)

P LAB

(%)

±0.320 ±2.2 ±3.240

±0.422 ±2.2 ±3.502

±0.478 ±2.2 ±3.578

±0.130 ±2.2 ±3.010

±0.139 ±2.2 ±3.079

±1.320 ±2.2 ±4.410

0 B 0

(%) (%) (%)

±8.060 1.915 -2.419

±8.422 2.091 -4.521

±8.178 2.556 -6.345

±7.750 1.863 -1.102

±7.899 2.049 -2.459

±9.270 2.106 -3.553

26
TABLE 8

Laboratory uncertainties, P

L ^ B

, total uncertainty of the

i ntercomparison U, and differences from NBS measurements D.

The other column headings are defined in Section VII-B.

Laboratory 1

Prismatic Retroref 1 ector

a gree)

01 (degree)

32

(degree) R

3x6R n

1-\ l"

)

\

L

(%)

P LAB

(%)

U

(%)

B

(%)

D

(%)

0.5 0 0

O TOO ±3. 109 ±1 .760 ±2.6 ±7 .469 ±14.85 -0.665 -0.522

0.5 -10 0 ±2.530 ±0.461 ±2.6 ±5 .591 ±13.02 -0.283 1.300

0.5 +10 0 ±3.772 ±0.776 ±2.6 ±7.148 ±15.11 -0.216 -0.858

0.2 0 0 ±3.200 ±0.212 ±2.6 ±6.012 ±13.64 0.281 -2.197

0.2 -10 0 ±3.918 ±0.381 ±2.6 ±6.899 ±15.03 -0.289 -3.666

0.2 +10 0 ±4.199 ±0.308 ±2.6 ±7.107 ±15.43 0.916 -2.244

0.5 0 -20 ±2.899 ±1.578 ±2.6 ±7.077 ±22.98 5.413 -20.27

0.5 -10 -20 ±4.637 ±2.683 ±2.6 ±9.920 ±26.62 2.148 -15.35

0.5 +10 -20 ±3.579 ±2.594 ±2.6 ±8.773 ±23.07 4.531 -10.30

0.2 0 -20 ±5.327 ±1.049 ±2.6 ±8.976 ±22.88 4.159 -16.66

0.2 -10 -20 ±11.24 ±2.738 ±2.6 ±16.58 ±32.88 3.736 -15.70

0.2 +10 -20 ±9.527 ±1.447 ±2.6 ±13.57 ±28.47 4.793 -10.96

0.5 0 +20 ±2.646 ±3.505 ±2.6 ±8.751 ±20.19 6.435 -5.590

0.5 -10 +20 ±3.206 ±3.626 ±2.6 ±9.432 ±22.03 1.137 2.566

0.5 +10 +20 ±2.982 ±1.651 ±2.6 ±7.233 ±19.03 7.990 -5.171

0.2 0 +20 ±2.565 ±2.452 ±2.6 ±7.617 ±16.94 6.363 -5.660

0.2 -10 +20 ±6.625 ±2.322 ±2.6 ±11.55 ±23.65 1.437 2.403

0.2 +10 +20 ±5.065 ±2.383 ±2.6 ±10.05 ±19.89 4.654 -3.812

27
TABLE 9

Laboratory uncertainties ^LAB » tota^ uncertainty of the

i ntercomparison U, and differences from NBS measurements D.

The other column headings are defined in section VII-B.

Laboratory 2

High Intensity White

a

;gree)

01 (degree)

*|(%)

R

3x6R n _J>(%)

R

P

L

(%)

P LAB

(%)

U

(%)

B

(%)

D

(%)

0.2 -4 ±0. 39 ±0.095 ±3.1 ±3.585 ±8.755 1 .102 -0.944

0 2 20 ±0.54 ±0.102 ±3.1 ±3.742 ±8.922 1 .038 -1 520

0.2 40 ±1 .353 ±0.121 ±3.1 ±4.574 ±9.794 0.769 -5 .485

1 5 -4 ±0. 38 ±0.137 ±3.1 ±3.617 +8 937 1 .075 -6 . 192

1 R ?nCVJ —U . J J ±2.698 ±3.1 +fi — U • J40 ^4ft + 11 f\l 1 . 1 X o —-At • 7Qf1 nu

1.5 40 ±1.20 ±0.359 ±3.1 ±4.659 ±9.799 0.612 -4.969

Engineering White

a gree)

ei (degree

)

R

3x6R n _E<%)

R

P

L

(%)

P LAB

(%)

U

(%)

B

(%)

D

[%)

0.2 =4 ±0.39 ±0.312 ±2.3 ±3.002 ±7.822 1.926 -0.497

0.2 20 ±1.002 0 ±2.3 ±3.302 ±8.222 2.214 -0.875

0.2 40 ±2.247 ±0.199 ±2.3 ±4.746 ±9.346 2.806 0.597

1.5 -4 ±0.43 0 ±2.3 ±2.730 ±7.470 2.121 -3.355

1.5 20 ±0.52 0 ±2.3 ±2.820 ±7.640 2.677 -7.040

1.5 40 ±1.281 0 ±2.3 ±3.581 ±8.441 2.969 -0.405

28
TABLE 10

Laboratory uncertainties Pj_^g, total uncertainty of the

i ntercomparison U, and differences from NBS measurements D. The other column headings are defined in Section VII-B.

Laboratory 2

Prismatic Retroref 1 ector

32 AR ,0/ 3x<5R LAB

gree) (degree) (degree) R (%) (%) (%) (%)

0.5 0 0 ±1.131 NR* ±2.1 ±3.231 ±10.61 "0.638 8.523

0.5 -10 0 ±1.341 NR ±2.1 ±3.441 ±10.87 -1.487 8.249

0.5 + 10 0 ±1.591 NR ±2.1 ±3.691 ±11.65 -0.482 10.07

0.2 0 0 ±1.360 NR ±2.1 ±3.460 ±11.09 -0.330 8.050

0.2 -10 0 ±1.862 NR ±2.1 ±3.962 ±12.09 0.310 8.360

0.2 + 10 0 ±2.189 NR ±2.1 ±4.289 ±12.61 0.591 8.837

0.5 0 -20 ±8.173 NR ±2.1 ±10.27 ±26.17 2.280 15.14

0.5 -10 -20 ±8.998 NR ±2.1 ±11.10 ±27.80 -1.103 11.30

0.5 + 10 -20 ±7.331 NR ±2.1 ±9.431 ±23.73 1.610 23.96

0.2 0 -20 ±6.502 NR ±2.1 ±8.602 ±22.50 2.906 14.75

0.2 -10 -20 ±8.659 NR ±2.1 ±10.76 ±27.06 -0.545 13.34

0.2 + 10 -20 ±8.517 NR ±2.1 ±10.62 ±25.52 1.366 17.11

0.5 0 +20 ±4.201 NR ±2.1 ±6.301 ±17.74 0.034 10.65

0.5 -10 +20 ±6.490 NR ±2.1 ±8.590 ±21.19 -2.161 8.488

0.5 + 10 +20 ±5.020 NR ±2.1 ±7.120 ±18.92 1.029 10.67

0.2 0 +20 ±3.203 NR ±2.1 ±5.303 ±14.62 0.198 6.51

0.2 -10 +20 ±7.440 NR ±2.1 ±9.540 ±21.64 -3.978 0.935

0.2 +10 +20 ±4.225 NR ±2.1 ±6.325 ±16.17 0.344 7.684

*Not Reported

29
TABLE 11

Values for Master Set of Filters Associated with NBS The column headings are defined in Section VI 1

1 -A. All values are expressed in absolute luminous transmi ttance units.

Reference

Filter Values A for Y ""MASTER ^ MASTER ISCC-NBS

Color Name

1-1 0.01432 0.0003 0.0005 0.0001 0.0009 Vivid Blue

1-2 0.06720 0.0003 0.0007 0.0001 0.0011 Deep Yellowish Green

1-3 0.68257 0.0003 0.0004 0.0004 0.0011 Vivid Orange Yellow

1-4 0.79587 0.0003 0.0003 0.0009 0.0015 Vivid Yellow

1-5 0.36190 0.0003 0.0005 0.0012 0.0020 Vivid Reddish Orange

1-6 0.19379 0.0003 0.0001 0.0006 0.0010 Vivid Red

1-7 0.00000 0.0000 0.0000 0.0000 0.0000 Black (Infrared

Transmitti ng)

30
TABLE 12

Uncertainties of the participating laboratories' measurements.

The headings are defined in Section VIII-B.

values are expressed in absolute luminous transmittance units.

Laboratory 1

i Iter 3x6Y

P

C U D D>U

II-l 0.0001 0.0001 0.0010 0.0003 No

1 1-2 0.0010 0.0002 0.0022 0.0013 No

1 1-3 0 0035 0.0008 0.0050 0.0137 Yes

1 1 -4 0.0035 0.0009 0.0050 0.0082 Yes

II-5 0.0052 0.0011 0.0071 | 0.0097 Yes

1 1-6 0.0048 0.0006 0.0058 0.0026 No

1 1 -7 0.0000 0.0000 0.0000 0.0000 No

Laboratory 2

ilter 3x6Y

P

C U D D>U

1 1

1 -1 0.0002 0.0001 0.0011 0.0001 No

1 1 1-2 0.0005 0.0000 0.0015 0.0001 No

1 1 1-3 0.0007 0.0003 0.0017 0.0098 Yes

1 1 1 -4 0.0010 0.0002 0.0018 0.0079 Yes

1 1 1 -5 0.0004 0.0010 0.0022 0.0053 Yes

1 1 1 -6 0.0002 0.0009 0.0015 0.0053 Yes

1 1

1 -7 0.0000 0.0000 0.0000 0.0000 No

31
ILLUSTRATION A Date

If more detailed information is available, please use separate sheets to report

that information. The individual values of luminous transmittance are to be reported on separate sheets of paper.

COLORED FILTERS

Set No.

Average

Luminous Standard Number of Filter No. Transmittance Devi at i on Measurements

1

2

3

4

5

6

7

Method used (check one)

Straight through

Diffuse reflector

Other (describe)

32
ILLUSTRATION B Date

PRISMATIC (CUBE CORNER)

If more detailed information is available, please use separate sheets to report

that information. The individual values of R are to be reported on separate sheets

of paper.

Sample Identification (A, B, C, etc.)?

Please set e = 0 and check here.

Source aperture (minutes of arc)? Uncertainty in source aperture?

Receiver aperture (minutes of arc)? Uncertainty in receiver aperture?

Uncertainty due to non-linearity or uncertainty in correction

for non-linearity?

Uncertainty in Bi? degrees

Uncertainty in B2? degrees

Uncertainty in a? degrees

Uncertainty in e? degrees

Observation distance m

33
ILLUSTRATION B (cont'd.)

Average

6 3 a R Standard Number of

l 2 (degrees) (degrees) (degrees) (cd/lx) Deviation Measurements

0 0 0.5

-10 0 0.5

+ 10 0 0.5

0 0 0.2

-10 0 0.2

+10 0 0.2

0 +20 0.5

-10 +20 0.5

+ 10 +20 0.5

0 +20 0.2

-10 +20 0.2

+ 10 +20 0.2

0 -20 0.5

-10 -20 0.5

+10 -20 0.5

0 -20 0.2

-10 -20 0.2

+10 -20 0.2

34
ILLUSTRATION C

BEAD SHEETING

If more detailed information is available, please use a separate sheet to report

that information. The individual values of R are to be reported on separate sheets

of paper.

High Intensity or Engineering White?

Please set e = 0 and 3 = 0 and check here.

2

Sample Identification (A, B, C, etc.)

Source Aperture (minutes of arc)? Uncertainty in source aperture?

Receiver Aperture (minutes of arc)? Uncertainty in receiver aperture?

Uncertainty due to non-linearity or uncertainty in correction

for non-linearity?

Uncertainty in 3 ? degrees

1

Uncertainty in a? degrees

Observation distance m

Average

3 a R Standard Number of

l (degrees) (degrees) (cd/lux) Deviation Measurements

-4 .2

20 .2

40 .2

-4 1.5

2U 1.5

40 1.5

35
References

1. Retroreflection--Definition and Measurement, Publication CIE No. 54(TC-2.3) 1982.

Available from U.S. National Committee, K. D. Mielenz, Secretary, National Bureau of Standards (Div. 534), Washington, O.C. 20234.

2. Colorimetry, Publication CIE No. 15 (E-l.3.1) 1971. Available from the same source as reference [1].

3. venable, W. H., Jr., and Johnson, N. L., "Unified Coordinate System for Retroreflectance

Measurements", Appl . Opt. J.9, 1234 (1980).

4. Eckerle, K. L., "Photometry and Colorimetry of Retroref 1 ecti on: State-of-the-Measurement

Accuracy Report", NBS (U.S.) Technical Note 1125 (July 1980).

5. Eckerle, K. L., and Hsia, J. J., "Proposed Standards for the NBS Retroref lection MAP",

COLOR, research and application ]_ (3), 235 (1982).

6. Eckerle, K. L., Hsia, J. J., and Liggett, W. S., Jr., "Geometrical Alignment Errors in

the Measurement of Prismatic Retroref lectors", to be published.

7. Mielenz, K.D., Eckerle, K. L., Madden, R.P., and Reader, J., "New Reference Spectrophotometer", Appl. Opt. 12 (7), 1630 (July 1973).

8. Mielenz, K. D., and Eckerle, K. L., "Design, Construction, and Testing of a New High

Accuracy Spectrophotometer", Nat. Bur. Stand. (U.S.) Tech Note 729 (June 1972).

9. Eckerle, K. L., "Modification of an NBS Reference Spectrophotometer", Nat. Bur. Stand.

(U.S.) Tech Note 913 (July 1976).

10. Eckerle, K. L., Hsia, J. J., Weidner, V. R., and Venable, W. H., Jr., "NBS Reference

Retroreflectometer", Appl. Opt. 19, 1253 (1980).

11. Weidner, V. R., and Hsia, J. J., "Reflection properties of pressed polytetrafluoroethylene

powder", J. Opt. Soc. Am. 71 (7) 856 (1981).

12. Johnson, N. L., and Stephenson, H. F., "Influence of Aperture Size on the Photometry of

Retroreflectors", Appl. Opt. 19, 1247 (1980).

13. Certain material is identified by manufacturer only to define the procedure. It implies in no way endorsement or evaluation by the National Bureau of Standards or that the material is

best for the procedure.

36
Appendix A. Measuring linearity by means of the light addition method

In reference [10], a method of measuring linearity using two triangular sheets of

retroreflecting material is described. This is a general method. Following are excerpts from

reference [10],

The linearity of the detector and electronics is measured using the light addition method.

Two 45° triangular pieces of retroreflecti ve sheeting with a 30.5-cm base and 30.b-cm height as shown in figure Al are illuminated at the sample carrier. A rotating semicircular baffle covered

with black cloth can block either triangle A or B or both at the same time. A measurement

sequence is automatically done: A + B, A, D, B, D, A, A + B, A, D, B, D, A, A + B, A, D, B, D, A, A + B, where A + B is the signal with both triangles illuminated, A is the signal with B masked by

the black cloth, D is the signal with A and B both masked, and B is the signal with A masked. The

average of each type of reading is taken. Signals A, B, and A + B must be corrected by

subtracting the contribution due to reflection from the black cloth. This contribution to signal

A is calculated by multiplying signal D by the ratio of black area exposed during reading A to the

black area exposed during reading D:

where a is the total black area during reading D, and a

a is the area of triangle A. The corrected

value A

1 is then derived from

A' - A - DA

= A - D • (

iT!i

)

and similarly for B and A + B. We may then define the fractional nonlinearity a at a signal level

(A ' + B')/2 as

/ A' + B' \ (A + B)' , 0

V 2 /

" A

1 + B

' "

The exposed area of the triangles is then reduced by automatically raising them behind a black

velvet cloth screen such that the signal (A + B) is reduced by a factor of 2. (It is not critical

that it be exactly two.) The measurement sequence stated above is repeated, and a new value of

o is calculated that gives the nonlinearity at approximately one-fourth of the original signal.

This process of stepping down is repeated until the signal is smaller than that which will be obtained from the test retroreflector. The nonlinearity AS at arbitrary signal S will be

AS = o(S)S. With the apparatus in current use, the correction AS never exceeds 30 parts in 30,000

at the upper end of the scale. Over the dynamic range normally used, the noise and nonlinearity

are indistinguishable. Thus, it is convenient to express linearity and noise together in

Table 4.

37
7 / m<

UJ>

LU

CO

UJ>o

cr

Ll.

CD

CD co r— S_ 4- O

cu -t-> s- o C CU O i— C M—

0)

cr> s- • cocu •.- s- +j

+->+-> fO

tO CD i—

+-> i- CL o

5- 5- >> 'i— fO

Ll. 3 C

Cn

O

c: •!- • • <T3 +-> >i-i- fO +J S- -l-> i- -t-> oo

(O

O Oi

cu 3 c

i— cu o

-c cu S- +J i— O <4-

-»-> .c cu U U S-

<L> -iC

+j x: o

cu 3 c

-o c - cn o o c

•i- CU

S- +J •«

(/) i— CU

ID D.-P

CU £= E « =3 UJ o

s- E o

ci- CU

CD S- 00 i— (O 3 o

+-> s- co

fO -r- s- o -o

it) •<-

C

D-E IB Q. CU <C co <c

CU

cn

33
This test need not be automated and may be done manually. We will try to provide here a few

practical hints. First of all, it is not necessary that the retrorefl ecti ng material be triangular although this simplifies obtaining small signal levels. Secondly, it is not necessary

to use sheeting; it should be possible to use prismatic retrorefl ectors

.

Generally, the two or more retrorefl ectors used should give a signal larger than that obtained

in the course of a measurement. This large signal is obtained when measuring the normal

illuminance of the source. Many commonly used detectors have been shown to have a non-linearity

which is wavelength independent, and it may be possible to make these measurements without the

photopic correction filter. If black cloth is used to partially block the light, it is good

practice to use an opaque backing since the retrorefl ector may reflect light back through the

pores of the black cloth.

39
Appendix B.

We will identify the manufacturers and type of the elements of

the MAP package for information purposes [13].

Retroref 1 ector

High Intensity Bead Sheeting

(Encapsulated Lens) (White)

Manufacturer

Mi nnesota Mi ni ng and

Manufacturing Co. (3M)

Engineering Grade Bead Sheeting

(Enclosed Lens) (White)

Same as above

Two-orientation-hexagon cube

(Prismatic Retroreflector)

(Col orl ess

)

Elgin Molded Plastics

Filter Type

R-60, Y-52, B-440

0-54, 0-58, G-530

IR-80

Manufacturer

Hoya Optics USA, Inc.

40
Appendix C.

A prototype report of calibration is included here to clarify the MAP procedure and to show

the results of a sample intercomparison. It includes the purpose, material, measurements,

results, and conclusions of a pilot run.

Report of Calibration

for the

Retroref lectance MAP Service for

Coefficient of Luminous Intensity

Reported to Laboratory 1 Somewhere, U.S.A.

(Please see your purchase order number XYZ dated August 9, 1983.)

1. Purpose .

The purpose of this test is to determine coefficient of luminous intensity for two bead

sheeting retroref 1 ectors and a prismatic cube-corner retrorefl ector. Also, the luminous transmittance of seven colored glass filters is determined. These determinations have been accom- plished by the NBS and the participating laboratory. Conclusions from these determinations are to

be presented.

2. Materi al .

The retroref 1 ectors and colored filters contained in the MAP package are described in

reference 1.

3. Measurements .

Measurements made by the NBS are described in references 1 through b. Measurements made by

the participating laboratory were performed using its normal operating procedures.

4. Results .

The results of the MAP comparison are shown in Tables 1 ana 2 for the retroref 1 ectors and

Table 3 for the colored glass filters. Since NBS measured the MAP package before and after the

participating laboratory made its measurements, three values are reported for each condition.

A. Retroreflectors.

In addition to the angular parameters a, 3i, and B2 we define some quantities relevant

to data analysis in Tables 4 and 5. AR

(1) The quantity 100 -=- is the upper bound of the uncertainty in R predicted by the uncerK tainties of the angular parameters expressed as a percent and estimated from data supplied by the

participant.

(2) The quantity 100

3

R X 6Rp

is three times the standard deviation of the mean R obtained

P from repeated measurements by the participant expressed in percent.

(3) The quantity L is the uncertainty in R due to aperture centroid (source and receiver),

linearity, and color temperature of the source and photopic response of receiver as shown in Table 6 and expressed in percent.

41
(4) The quantity P^g is the sum of (1), (2), (3). It is an upper bound of the uncertainty

in R due to the participating laboratory and is expressed in percent.

(b) The quantity U is the sum of P LAB ' P NBS ( tne u PP er bound on the uncertainty due to NBS - see reference 1), and a third component which estimates changes in the retroreflectance of the MAP sample due to possible temperature and pressure changes in its environment.

(6) The quantity B is the percent change in the measurements made by NBS before and after the

measurements performed by the participating laboratory.

(7) The quantity D is the percent difference between the NBS average of before and after

measurements and the value obtained by the participating laboratory.

B. Colored Filters.

The bounds on the uncertainties of the luminous transmittance values for the colored glass

filters are listed in Table 7.

(1) The quantity 3 x 6Yp is three times the standard deviation of the mean.

(2) The quantity C is the difference between the Y values obtained by NBS before and after

the participating laboratory.

(3) The quantity U is the sum of items (1) and (2) and bounds on the uncertainties due to

NBS. (See reference 1.)

(4) The quantity D is the difference between the participant values and the NBS values.

5. Conclusion .

If D > U; then there is a bias or systematic error.

(A) Retroref 1 ectors

.

Since D < U in Tables 1 and 2 for both the bead sheeting retroref 1 ectors and the

prismatic retroref lector, we conclude that the estimates given by the participant used to obtain

M are valid. Therefore, the quantities M are realistic bounds on the participant's measurement

process

.

(B) Colored Glass Filters

Since D > U in Table 4 in some cases, we conclude that there exists a bias in three

cases. It is necessary for the participant to decide if these biases are of an acceptable level.

In the other cases, we conclude that the bias of the participating laboratory is equal to or less

than U.

For the Di rector,

Radiometric Physics Division

Center for Radiation Research

42
Appendix C

TABLE 1

Results of pilot i ntercomparison with Laboratory 1.

R LAB are tne resu l ts of the participating laboratory,

with R§bs°

RE and R NBSER the NBS measurements made before

and after the participating laboratory.

Laboratory 1

High Intensity White

a (degree) (degree) BEFORE

NBS R LAB rAFTER

NBS

0.2 -4 4.0854 4.110 4.0048

0.2 20 3.8072 3.817 3.7393

0.2 40 2.4309 2.407 2.4065

1.5 -4 0.1991 0.203 0.1960

1.5 20 0.1914 0.196 0.1904

1.5 40 0.1371 0.139 0.1342

Engineering White

0.2 -4 1.6180 1.626 1.5572

0.2 20 1.2014 1.230 1.1522

0.2 40 0.3491 0.362 0.3317

1.5 -4 0.1340 0.133 0.1291

1.5 20 0.1245 0.125 0.1195

1.5 40 0.0776 0.0787 0.0744

43
Appendix C

TABLE 2

R LAB are tlie ^s^ts °f tne participating laboratory,

with R§BS°

RE and R NBSER the NBS n^asurements made

before and after the participating laboratory.

Laboratory 1

Prismatic Ref 1 ector

a?gree) (degree)

32

(degree)

BEFORE

NBS R LAB nAFTERNBS

0.5 0 0 0.9042 0.915 0.9163

0.5 -10 0 0.8820 0.873 0.8870

0.5 + 10 0 0.8548 0.864 0.8585

0.2 0 0 5.5970 5.704 5.5657

0.2 -10 0 4.7419 4.930 4.7694

0.2 +10 0 4.7238 4.786 4.6381

0.5 0 -20 0.3690 0.421 0.3311

0.5 -10 -20 0.2949 0.333 0.2825

0.5 +10 -20 0. 3080 0. 325 0.2813

0.2 0 -20 2.1500 2.408 1.9783

0.2 -10 -20 1.9689 2.196 1.8271

0.2 + 10 -20 2.0741 2.196 1.8842

0.5 0 +20 0.4284 0.425 0.3766

0.5 -10 +20 0.3114 0.300 0.3044

0.5 + 10 +20 0.3717 0.362 0.3167

0.2 0 +20 2.8327 2.814 2.4938

0.2 -10 +20 2.0371 1.960 1.9794

0.2 + 10 +20 2.5243 2.504 2.2998

44
Appendix C

TABLE 3

Results of pilot i ntercompari son with Laboratory 1.

Y LAB are tne resu ^ ts of the participating laboratory,

with Yjj|£ 0RE and y{^er the NBS measurements made before

and after the participating laboratory.

All results are expressed in units of absolute luminous transmittance,

Filter

1-1

1-2

1-3

1-4

1-5

1-6

1-7

yBEFORE

NBS

Laboratory 1

Y

0.01300

0.06384

0.67456

0.79876

0.35934

0.19549

0.0000

LAB

0.01338

0.06270

0.68900

0.80600

0.36800

0.19750

0.00001

YAFTER

NBS

0.01317

0.06419

0.67611

0.78694

0.35723

0.19438

0.0000

45
Appendi x C

TABLE 4

Laboratory uncertainties P|ar» total uncertainty of the

i ntercompari son U,and differences from NBS measurements D.

The other column headings are defined in the text.

Laboratory 1

High Intensity White

a Bi

egree ) (degree)

^-(%)

R

3x6R„

£(%)

R

P

L

(%)

P LAB

(%)

U

(%)

B

(%)

D

(%)

0.2 -4 ±0.86 ±0.421 ±2.3 ±3.581 ±8.751 0.997 -1.604

0.2 20 ±0.89 ±0.272 ±2.3 ±3.462 ±8.642 0.900 -1.159

0.2 40 ±1.052 ±0.432 ±2.3 ±3.784 ±9.004 0.505 0.484

1.5 -4 ±0.95 ±0.085 ±2.3 ±3.335 ±8.655 0.785 -2.759

1.5 20 ±0.98 ±0.530 ±2.3 ±3.810 ±9.130 0.262 -2.672

1.5 40 ±0.97 ±0.124 ±2.3 ±3.394 ±8.534 1.069 -2.470

Engi neering White

a 3i

egree ) (degree) R

3x6R n _R<%)

R

P

L

{%)

P LAB

(%)

U

(%)

B

(%)

D

(%)

0.2 -4 ±0.72 ±0.320 ±2.2 ±3.240 ±8.060 1.915 -2.419

0.2 20 ±0.88 ±0.422 ±2.2 ±3.502 ±8.422 2.091 -4.521

0.2 4U ±0.90 ±0.478 ±2.2 ±3.578 ±8.178 2.556 -6.345

1.5 -4 ±0.68 ±0.130 ±2.2 ±3.010 ±7.750 1.863 -1.102

1.5 2U ±0.74 ±0.139 ±2.2 ±3.079 ±7.899 2.049 -2.459

1.5 40 ±0.89 ±1.320 ±2.2 ±4.410 ±9.270 2.106 -3.553

46
Appendix C

TABLE 5

Laboratory uncertainties, » tota l uncertainty of the

i ntercompari son U, and differences from NBS measurements D.

The other column headings are defined in the text.

Prismatic Retroreflector

a 01 B2 —(%)3x6R n

L P| AD LAB U B D

jgree) (degree

)

(degree

)

R R

P [%) (%) (%) (%) (%)

0.5 0 0 ±3.109 ±1 .760 ±2.6 ±7.469 ±14.85 -0.665 -0.522

0.5 -10 0 ±2.530 ±0.461 ±2.6 ±5.591 ±13.02 -0.283 1.300

0.5 +10 0 ±3.772 ±0.776 ±2.6 ±7.148 ±15.11 -0.216 -0.858

0.2 0 0 ±3.200 ±0.212 ±2.6 ±6.012 ±13.64 0.281 -2.197

0.2 -10 0 ±3.918 ±0.381 ±2.6 ±6.899 ±15.03 -0.289 -3.666

0.2 +10 0 ±4.199 ±0.308 ±2.6 ±7.107 ±15.43 0.916 -2.244

0.5 0 -20 ±2.899 ±1.578 ±2.6 ±7.077 ±22.98 5.413 -20.27

0.5 -10 -20 ±4.637 ±2.683 ±2.6 ±9.920 ±26.62 2.148 -15.35

0.5 +10 -20 ±3.579 ±2.594 ±2.6 ±8.773 ±23.07 4.531 -10.30

0.2 0 -20 ±5.327 ±1.049 ±2.6 ±8.976 ±22.88 4.159 -16.66

0.2 -10 -20 ±11.24 ±2.738 ±2.6 ±16.58 ±32.88 3.736 -15.70

0.2 +10 -20 ±9.527 ±1.447 ±2.6 ±13.57 ±28.47 4.793 -10.96

0.5 0 +20 ±2.646 ±3.505 ±2.6 ±8.751 ±20.19 6.435 -5.590

0.5 -10 +20 ±3.206 ±3.626 ±2.6 ±9.432 ±22.03 1.137 2.566

0.5 + 10 +20 ±2.982 ±1,651 ±2.6 ±7.233 ±19.03 7.990 -5.171

0.2 0 +20 ±2.565 ±2.452 ±2.6 ±7.617 ±16.94 6.363 -5.660

0.2 -10 +20 ±6.625 ±2.322 ±2.6 ±11.55 ±23.65 1.437 2.403

0.2 +10 +20 ±5 . 065 ±2.383 ±2.6 ±10.05 ±19.89 4.654 -3.812

47
Appendix C

TABLE 6

Other uncertainties associated with measurements of R for the

participating laboratories.

Laboratory 1

Engineering Grade High Intensity Prismati

Aperture Centroid

(Source and Receiver'

Li nearity

Color Temperature of

Source and Photopic

Response of Receiver

White

±0.1%

±2.0%

±0.1%

White

±0.2

±2.0%

±0.1%

CI ear

±0.5%

±2.0%

±0.1%

2.2% 2.3% 2.6%

TABLE 7

Uncertainties of the participating laboratories' measurements.

The headings are defined in Section VIII-B.

All values are expressed in absolute luminous transmittance units

Laboratory 1

Filter 3x6Y

p C u D D>U

1

1 -1 0.0001 0.0001 0.0010 0.0003 No

II-2 0.0010 0.0002 0.0022 0.0013 No

1 1-3 0.0035 0.0008 0.0050 0.0137 Yes

1 1 -4 0.0035 0.0009 0.0050 0.0082 Yes

1 1 -5 0.0052 0.0011 0.0071 0.0097 Yes

1 1 -6 0.0048 0.0006 0.0058 0.0026 No

1 1-7 0.0000 0.0000 0.0000 0.0000 No

43
Appendix C

References

.

(1) Retroref 1 ectance MAP Service for Coefficient of Luminous Intensity, K. L. Eckerle and

J. J. Hsia, to be published as an NBS Special Publication.

(2) NBS reference retroref lectometer, K. L. Eckerle, J. J. Hsia, V. R. Weidner, and

W. H. Venable, Jr., Appl . Opt. 1_9, (8) 1253 (1980).

(3) New Reference Spectrophotometer, K. D. Mielenz, K. L. Eckerle, R. P. Madden, and

J. Reader, Appl. Opt. 12 (7) 1630 (1973).

(4) Proposed Standards for the NBS Retroref 1 ecti on MAP, K. L. Eckerle, and J. J. Hsia,

COLOR research and application 1_ (3) 235 (1982).

(5) Geometrical Alignment Errors in the Measurement of Prismatic Retroref 1 ectors

,

K. L. Eckerle, J. J. Hsia, and W. S. Liggett, Jr. (accepted for publication in COLOR research and application).
NBS-114A (REV. 2-8C) U.o. LJ c.rJ |. V_)r v»* iVl ivl

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1 PI I RI 1 fATlON

REPORT NO.

NBS SP 671

OR 0H. a Pprfnrm I " 1 IUI IM inII IEo Hroan W | £CII l a R pnnrtl No1 ^ Vj o 3. Publication Date

February 1984

4. TITLE AND SUBTITLE

Retroreflectance MAP Service for Coefficient of Luminous Intensity

5. AUTHOR(S)

Kenneth L. Eckerle and Jack J. Hsia

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Same as item 6.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 84-601001

~2 Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bibliography or literature survey, mention it here)

This publication is written for those participating in the retroreflectance

measurement assurance program (MAP) service provided by the National Bureau of

Standards. This service is to verify the accuracy of measurement of coefficient

of luminous intensity (R). This paper presents the techniques and procedures

that are pertinent to participating in the MAP service, as well as a detailed

explanation of the error analyses. Uncertainties for both retroreflectance and

luminous transmi ttance were determined from two pilot studies carried out with

the assistance of two industrial laboratories and by research performed on the

elements of the MAP package.

12. KEY WORDS (Six to tweive entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

bead sheeting retroreflector; coefficient of luminous intensity; diagnostic filters; error estimation; experimental design; luminous transmittance; Measurement Assurance

Program; prismatic retroreflector; uncertainty estimation.

13. AVAILABILITY

[X] Unlimited

| | For Official Distribution. Do Not Release to NTIS

[X] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

56

15. Price

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